High-frequency circuits



June 28, 1949. J. E. YOUNG HIGH-FREQUENCY CIRCUITS Filed OCb. 16, 1945 4 ATTORN EY Patented June 28, 1949 UNITED ariane HIGH-FREQUENCY CIRCUITS John E. Young, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 16, 1945, Serial No. 622,603

7 Claims. (Cl. Z50-36) My present invention relates to high frequency apparatus and deals particularly with low power and high power vacuum tube amplifying systems for short waves. i

In short wave, high power transmitters such as used for frequency modulation broadcasting at carrier frequencies lying, for example, between '78 and 104 megacycles, it has been proposed to use transmission lines as interstage coupling links for the relatively low power driving amplifiers. Transmission lines for this purpose, however, are objectionable since they are bulky and, therefore, require a large amount of space. Moreover, they areinconvenient to tune or adjust. Conventional parallel tuned circuits between the driver stages are impractical in view of the relatively high stray input and output capacities of pentode tubes, preferably used in these stages because of their relatively high gain. That is, the stray capacities are `of such value that the inductances necessary to tune them at the higher frequencies are very small. As a result, the coil structures required to tune the stray capacities will be so small as not to be feasible if conventional parallel tuned circuit practice is followed.

To overcome the foregoing objections and more specifically to provide a driver-amplifier system which makes use of coils of practical size, despite stray capacities and operation at very high frequencies, is one object of my present invention. In this connection it should be noted that the invention provides for and enables tuning over a wide range of frequencies with ease.

A further object of my invention is to provide an improved high power amplifier system especially adapted for use at the higher frequencies. In carrying out this aspect of my invention, I make use of grounded grid power amplifier stages and circuits therefor which are especially free of, and the tendency to produce, parasitic oscillations. Also, I provide an improved cascaded arrangement of grounded grid amplifiers and circuits therefor which will develop substantial amounts of power at very short wave lengths.

A more detailed description of my present invention will now be given with the aid of the accompanying drawing wherein the single figure is a wiring diagram of a frequency doubler and low power driver amplifier for a frequency modulation transmitter, the circuits therefor being such as to enable the use of tuning coils of practical size despite high stray tube and other capaoities and despite operation at very high frequencies.

Referring to the frequency doubler, lower power amplifier of the drawings, it will be noted that the arrangement is designed to double the frequency of the input derived from the frequency modulation exciter unit. The frequency modulated carrier input fed to the doubler 3 may lie anywhere in the range of from 39 megacycles to 54 megacycles with a maximum frequency swing of 37.5 kilocycles. The frequency doubled output is fed through low power amplifier i8 to utilization means such as an high power amplifier (not shown) feeding an antenna (not shown). The high power amplifier may be as disclosed in my divisional application `Serial #723,425, filed January 21, 1947. Because of the frequency doubling produced by tube 8 the antenna is then supplied with waves having a mean frequency lying at any desired point in the band of from '78 megacycles to 108 megacycles with a maximum frequency swing, in any case, of kilocycles. The latter is the maximum swing specifled for FM broadcasting at the present time by the Federal Communications Commission.

As shown, frequency modulated output from the exciter is fed to the frequency doubler by way of a concentric line system having an external grounded conductor 2 and an inner conductor 4. Inner conductor l is connected to the grid 6 of doubler 8 through the variable tuning coil l@ and blocking condenser l2. The latter has negligible reactance at the operating frequencies.

Stray input capacity preceding coil if! is indicated bythe dotted capacitor M, and the stray output capacity for doubler 3 is indicated by the dotted condenser i6. Similarly, the stray input and output capacities for amplifier It, which serves to amplify the output of doubler il, are indicated respectively by dotted line condensers 2l) and 22 respectively.

At the frequencies involved, it has been proposed to use transmission lines, having substantially uniformly distributed inductance and capacity, as radio frequency circuits to be connected directly tofthe input and output terminals of doubler 8. Similarly, it has been proposed to use such lines as the contiguous radio frequency input and output circuits for amplilier I8. Transmission lines for these purposes, however, have several disadvantages. They do not lend themselves to compact construction;`

they are expensive and, in addition, they are not easily adjustable or tunable, which operation is desirable in order to compensate for vacuum tube variations. Also, it follows that trans- 'i stages. lpacitiesY have such low reactance at ithe fre-"f quencies involved that coil structures-required. Yto tune the stray capacities Wouldhaye" to'be output Ycapacities are relatively large at the frequencies under consideration, conventional YparallelQtuned circuits,Y consisting of coils and condensers in parallel* are not suitable as couplings or wave transferring means between the This follows since the stray shunt caof Very low inductance, resulting in-,coils oi'iinry practical size. u In accordance with one aspect of the present invention, the difculties and objections c on-v comitant to the use of transmission lines or conventional circuits between tube stagesare overcome by the employment of coupling circuits which make use ofl the stray capacities in such a way as to allow ofthe use of coils of `practical size. The latter are highly desirable sincethey are easily adjusted or variedfor tuning purposes. Furthermore, the present invention provides the proper coupling-means to `obtain the correct voltl ages at the desired' frequencies fortransfer on` to` succeeding stagesofthe system. A further Virtue of the 'present arrangement resides in the fact that by means of-a single tuning control or adjustmentthe system may be tuned-overia1widel range of frequencies such as a range from '78 to 108' megacycles;

Referring again Ato 'the drawing, the output impedance of transmission line 2; 4; asmeasured between theleft hand terminal of Coilv It. and ground, is of they order of 70' ohms. f to step up this impedance'to match theinput impedance ofthe. doubler tube- 8, which impedance as measured between the grid 6 and-ground 28 may be, 'roughly-'oftheorder of 10,600 ohms. Byconnecting coil lilas shown, this 'step-up in impedance is effected: In thisregard itlshould'be notedthat coil l'i'places stray -capacity i4 effectively in series with :the strap input capacity i3 of tube 8 at thedesired operating frequency.

Coil lll maybe rmade in the-formV` of a spiral coilhav'ing dimensions such as, roughly, one inch in diameter andfourl inches in length.l The coil maybe formed* of 'from' six to a dozen turns or silver plated; copper ribbonconductorfhaving a` It is desiredwidth of about one-quarter of an inch. This isa practical size forthe coil land it may also be madev adjustable as indicated diagrammatically in 'the' drawing, through the Y by the arrow; shown same; i

Resistor 26 is' a grid leak resistor. Choke coii lll-and ley-'pass condenser I1' areprovided to prevent `flowrv of'-radio'frequency currents 'inlleak resistor 2t. circuit, an ammeter 28 'shuntedy by a radio frequency by-pass condenser '35).4 In the cathode circuiti there is provided cathode return resistor.

32 for imparting an' initialnegative bias tothe gridt with respect tov the rcathode 34. Resistor 32, shunted byA a suitablebylpass condenser 38, also servesA as avprotective'resistorin the event that excitation fails.

Between plate38- of doubler' sand ground 24 the impedance'may be roughly of the-order of 50d!) ohms.` This'y is matched to the input impedance of amplier"l8,'having a value of the order;

of, roughly, 2000 ohmsyby'means of the coil 40, direct current-blockingc'ondenser 42 and Ichokecoil-s' 434 and :llligftlie"former"v of whichfv feeds 'plate There is also'provided in the grid" voltage to the plate 38 of the doubler 8. The latter, namely, choke 46, is in series with the leak 58 'for the grid 52 of amplier tube i8. e

Choke coils 44 and 4t may each have values of the order of l0 microhenries. One eiect of coils and 45 is to isolate the D. C. circaiits from the alternating radio frequency circuits. In addition; they' are designed sothat thelagging current which they draw partially compensates the leading current drawn by condenser 2l). In this the chokes serve to effectively increase the capacitive reactance of the input circuit 0f tube i8; f 'In the plate circuit of tube 8, there is also :provided a de-counl-.ing resistor 5t and an ammeter 55 byLpas'sed yby radio frequency by-pass condenser 58. There is also provided in the grid circuit of tube I8, ammeter t8 shunted by by-pass condenser 62. In the cathode return circuit for cathode es of pentode amplifier tube i8, there is provided the cathode resistor 5,8 shuntedrby by--pass condenser G8;

The stray capacity from plate 33, of doubler 8,

to ground is represented by the dotted line condensery it and',-similarly, thestr'ay capacity between control grid 52 and groundis'represented The'straycai pacity from the. plate HJ oftubeV i8 to' ground bythe dotted line condenser '20;

is representedby the dotted-linev condenser- 22- The effect of theastray capacity 2D `isto 'make dinicult, as a practical" matter, the,I building up" of Vdesiredradio frequencyvoltages.-Y By the useA of the coil-40', straycapacityZus, at the operating frequencies,*placedeffectively'in series with-x stray capacity IS whereby the effective capacity tobe resonated is reduced from', thing of for example, somethe orderfof `15 mmf. to something of the of tube Y2l through transmissionlinefv y2, 4."

It is emphasizedvthat with the arrangement shown, the stray outputcapacity tof tub'e8 is placed effectively in series with capacityf of 'tube I8 thereby making the effective output and input vreactancesl larger Aat the 'operating frequencies and thereby more easily resonated by practical c'oil 'structures such? as coil 40.1 Chokes 44`and '46 areY so'proportioned that" the net "capacity .throughoutthef outputinput circuits fori tubes 8` and |81isf' correct for` capacity is such, also, as to provide'proper impedance transformation'between the output terminals of tube 8 'andthe input terminalsoff By-pass condensers/45, 4l connected" to the lower ends of chokes i4 and Misupplyi' paths for passage of Vradio frequencylcurrents through the chokes.

Voltages are'fedto the plates 3B; I0 of the pentode doubler 8 and' pentode` amplier I8 from` a suitable rectifier'88'and'connecting' lead 82. By the action vof voltage dropping' resistor 84, thevoltage from source 80 is'dr'opped' to Ia suitablev value to be fed'to the'screen guides 851,:

88 of tubes 8 andi-l8r These screen Igrids'are` provided" with radioY frequencyy by-pass condensers 9i?, 92 connected asI shown. The sup'- pressor electrodes of tubes- 8, I8 are connected f directly to the indirectly heatedf'cathodes 34, 64` Iwithin' the tube envelopes.

the stray 'input'y 'Condensers SI)I and 92 may be madel 'adjustable'to'series'tunei.v the inductances of the screen grid leads so'thatl the screen grids are maintained at groundA radio frequency potential.

The output of amplifier I8 is fed to utilization means by way of a circuit involving variable coil |02, blockingcondenser H2, choke |08 and a concentric transmission line H4, H8. The outer conductor Hi! of the concentric line is groundn ed by connection H6 and the internal conductor H8 coupled by a loop (not shown) to utilization means. Coil |62 is similar in construction to coil Ill. It, together with choke Hi8, is designed so as to give the correct impedance transfer between the output terminal of amplifier I8 and the input terminal of concentric conductor system H4, H8. The transmission line I I4, IIB is used in order that the utilization means may be located at any convenient or desired distance from the lower power driving am-` plifier I8. For example, a loop may be coupled to system H4, H8 and tothe rst amplifier stage in my divisional application Serial No. 723,425, filed January 21, 1947. The lower end of choke coil Hi8 is connected directly to ground for radio frequency currents by means of bypass condenser IOS. Variable condenser H3 connected as shown enables further adjustment of loading.

I-Iaving thus described my invention, what I claim is:

1. High frequency apparatus comprising a two conductor transmission line, a vacuum tube of the pentode type having an anode, a cathode, a grid, a screen grid and a suppressor grid, the suppressor grid being directly connected to said cathode, a connection from one output terminal of said line to the grid of said pentode tube, said connection including in series a i'lrst variable high frequency inductance coil and a blocking condenser, a low impedance connection from the other output terminal of the line to the cathode, circuits for subjecting said anode, cathode, grid and screen grid to suitable operating potentials, and a resonant output circuit connected between said anode and cathode, said output circuit comprising a second variable high frequency coil in series with the anode, said rst and second coils operating in conjunction with the stray grid to ground and plate to ground capacities respectively of said pentode tube at high frequencies, and means including a choke coil connected in series with said second mentioned coil for supplying plate potential to the anode of said pentode.

2. High frequency apparatus comprising a two conductor transmission line having relatively low output impedance, a vacuum tube of the pentode type having an anode, a cathode, a grid, a screen grid and a suppressor grid, the suppressor grid being directly connected to said cathode, a connection from one output terminal of said two conductor line to the grid of said pentode tube, said connection including in series a first variable inductance coil and a blocking condenser, a low impedance connection from the other terminal of the line to said cathode, circuits for subjecting said anode, cathode, grid and screen grid to suitable operating potentials, said circuits including a choke coil connecting a source of direct current potential to the control grid of said tube, and an harmonically tuned output circuit connected ben tween said anode and cathode, said harmonically tuned output circuit comprising a second variable inductance coil in series with the anode, the inductance of said coil being modified by the stray plate to ground capacity of said pentode tube, and

means including a choke connected in series with- 1 said coil for supplying plate potential to the anode of said pentode.

3. Apparatus as recited in claim 2, characterized by the fact that said transmission line is in trol grid, means for subjecting the control grid and cathode of said tube to high frequency voltages, a high frequency amplifier comprising an electron discharge device having an anode, a cathode and a grid, a variable inductance coil and a blocking condenser connected in series between the anode of said tube and the grid of said device thereby effectively connecting in series the stray output capacity of said tube and the stray input capacity of said device, a first choke coil connected to a terminal of said variable inductance coil removed from the anode of said tube, a source of anode potential connected to the other terminal of said choke coil and to the cathode of said tube, a second choke coil connected between the grid and cathode of said device, an output circuit connected between the anode and the cathode of said device, said choke coils with the variable inductance coil and the stray output capacity of said tube and the stray input capacity of said device being such as to cause resonance at a desired high frequency.

5. In high frequency apparatus, a vacuum tube frequency multiplier, said multiplier having an anode, a cathode and a control grid, means `for subjecting the control grid and cathode to high frequency exciting waves, a high frequency amplier comprising a vacuum tube having an anode, a cathode and a grid, a variable inductance coil and a blocking condenser connected in series between the anode of said multiplier and the grid of said amplifier thereby effectively connecting in series the stray output capacity of said multiplier and the stray input capacity of said amplifier, a first choke coil connected to a terminal of said variable inductance coil removed from the anode of said multiplier, a source of anode potential connected to the other terminal of said choke and to the cathode of said multiplier, a second choke coil connected between the grid and cathode of said amplier, an output circuit connected/between the anode and cathode of said amplifier, said choke coils with said variable inductance coil and the stray output capacity of said multiplier and the stray input capacity of said amplifier being such as to resonate at a desired harmonic frequency of the waves exciting the input electrodes of said multiplier.

6. High frequency apparatus comprising a concentric conductor transmission line, a pentode tube frequency multiplier, said tube having an anode, a cathode, a screen grid, a suppressor grid and a control grid, a blocking condenser and a `first variable coil connecting the control grid and the inner conductor of said line in series, means grounding the cathode of said multiplier and the outer conductor of said transmission line, the only effective capacitive reactance in the input circuit of said multiplier being the stray output capacity of said line and the stray input capacity of said tube, circuits for subjecting the screen grid and control grid of said multiplier to suitable operating potentials, a pentode tube amplifier, said tube amplifier having -a plate, a cathode, a control: gridaaascreenngrd:zandzafsuppnessor grid; a second vf variable f .coilvand 1 a blocking, condenser connected in series between the anodexof said multiplier'fand thefcontrolfgrid. of A said: amplier, af-rst '.choke yinfseries,` witluf-saidxsecond variable coil-conneetingralsouree'bf; anode :potential to the anode*Lofsaidmultiplierga iseeondfchoke coil connested between vthe#control-:grid of said amplifier and the cathode thereof, the only eective capaci'tiivey reaetancesr:` nlwthe :coupling circulit'4 between vsaid multiplier andffamplifler being. the stray outputA capacity-fof said multiplier and the stray-input capaoltyi ot saidV ampliier,` bot-h of` said stray: `capacities "vbeingreffectively" in series i 'by virtue aofythe,,connection-of said variable coil to'fsaiclanode;safid'couplingireuit including said y neeted between a terminator-'said coil removed from the' "plate offzsaid Aamplifier and saidy source tat-anode potential; c,

necting the output'eireuitfof-said4 amplifier tola twolconduetor.transmission line, one of -said conm `duotors ofsaiel transmission line being connectedvt to said third variable coil by a blocking condenser-,p

they other conductor-of saidlast mentioned transmission line being grounded;

J OI-IN E. YOUNG.

REFERENCES CITED The following referenices are of record in the-i Number Name Date 2,084,475 Braden June 22, 1937 2,220,437 Zotter Nov. 5, 19401A 2,223,736v Mertz Dec. 3, 1940 2,264,879 Heinecke` Dec. 2, 1941 2,272,062 George Feb. 3, .1942 i 2,310,455 Mullerv Feb. 9, 1943 2,352,455 l ISummerhayes June 27, 1944-11 2,406,739 Brown Sept. 3, 1946** 

